Tuning of packaged film bulk acoustic resonator filters

ABSTRACT

Packaged film bulk acoustic resonators may be tuned after packaging by exposing them to irradiation. In one embodiment, transparent covers may be provided so that the film bulk acoustic resonator filter may be exposed to laser irradiation to either add or remove material from the filter and to thereby adjust its frequency.

BACKGROUND

[0001] This invention relates generally to radio frequency filters,including film bulk acoustic resonators (FBARs).

[0002] Film bulk acoustic resonators have many advantages over othertechniques, such as surface acoustic wave (SAW) devices and ceramicfilters, particularly at high frequencies. For example, SAW filtersbegin to experience excessive insertion losses above 2.4 gigahertz andceramic filters are much larger in size and become increasinglydifficult to fabricate at increased frequencies.

[0003] A conventional FBAR filter may include two sets of FBARs toachieve the desired filter response. The series FBARs have one resonantfrequency and the shunt FBARs have another resonant frequency. Thefrequency of an FBAR is mainly determined by the thickness of itspiezoelectric film, which approximately equals a half wavelength of theacoustic wave. The frequencies of FBARs need to be precisely set toachieve a desired filter response.

[0004] For example, for a 2 gigahertz FBAR, the thickness of thepiezoelectric film may be approximately 2 microns. A one percent erroror variation in piezoelectric film thickness may change the frequency ofthe filter by approximately 20 megahertz, which is not acceptable if afrequency precision or accuracy of 16 megahertz is required.

[0005] A variety of techniques are known for adjusting the frequency ofan FBAR filter. However, the frequency of an FBAR filter may shiftduring or after packaging.

[0006] Therefore, there may be a need to tune the frequency of FBARfilters that were tuned prior to packaging, again after packaging.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is an enlarged cross-sectional view of one embodiment ofthe present invention;

[0008]FIG. 2 is an enlarged cross-sectional view of another embodimentof the present invention;

[0009]FIG. 3 is an enlarged cross-sectional view of another embodimentof the present invention; and

[0010]FIG. 4 is an enlarged cross-sectional view of another embodimentof the present invention.

DETAILED DESCRIPTION

[0011] Referring to FIG. 1, a film bulk acoustic resonator 10 mayinclude a silicon substrate 26 having a cavity 33 formed therein. Apiezoelectric film 32 may be formed over a lower electrode 24 and underan upper electrode 22. That structure may then be enclosed beneath atransparent cover 12 and above a transparent cover 30. Contacts 28 maybe provided for electrically connecting to the upper and lowerelectrodes 22 and 24. A sealing material 14 seals the cover 12 to therest of the film bulk acoustic resonator 10.

[0012] A tuning material 18 may be deposited on the inner surface of theupper transparent cover 12. In one embodiment, the material 18 may bepatterned as an array of dots. After the filter 10 has been fabricatedwith a frequency higher than the design or intended value, the frequencyof the filter 10 can be tuned to the desired value by decomposing thetuning material 18. In one embodiment, a laser (laser 1) may be utilizedto expose all or part of the tuning material 14, to decompose thatmaterial, and to cause it to be deposited on the upper electrode 22through the cavity 20.

[0013] The filter 10 frequency is then altered due to the mass loadingeffect or the change of the affected filter thickness. The tuningmaterial may be a non-conductive material, such as Al₂O₃, to avoid anyelectrical shorting.

[0014] The size of the spot created on the material 18 by the laser maybe smaller than the size of the tuning material 18 so that scanning isneeded to cover the full device. Alternatively, the size of the spot maybe larger, so only one shot is needed to finish the tuning of eachfilter 10.

[0015] Referring next to FIG. 2, in accordance with another embodimentof the present invention, material may be removed from the upperelectrode 22 to tune the filter after packaging. In this case, thefilter 10 a may be formed with a target frequency lower than the designvalue. Laser light is shined through the upper cover 12 and focused onthe top surface of the filter electrode 22. The filter 10 a can be tunedto the desired value by removing part of, or thinning, the top electrode22 of the filter 10 a. Alternatively, an additional layer 23 may bepositioned on top of the electrode 22, which then may be selectivelyremoved.

[0016] The size of the laser spot on the electrode 22 may be smallerthan the size of the electrode 22 so that scanning is needed to coverthe entire electrode 22. Alternatively, the size of the laser spot maybe larger, so that only one shot is needed to finish tuning each filter10 a.

[0017] Referring next to FIG. 3, an approach using both addition andremoval of material for tuning is illustrated. In this case, two lasers,laser 1 and laser 2, may be used so that both addition and removal ofmaterial can be applied. In this example, laser 1 is used for additionof material from the material 18 to the electrode 22. The laser 2 isused for removal of material from the electrode 22.

[0018] Two different frequencies of laser may be used for this purpose.Advantageously, in one embodiment, the tuning material 18 may betransparent to the light generated by the laser 2, but is absorbed bythe light generated by the laser 1. Alternatively, the material 18 maybe patterned or positioned to permit the laser 2 to pass withoutaffecting the material 18 while the laser 1 is aimed to hit thepatterned or positioned tuning material 18.

[0019] Referring to FIG. 4, in accordance with another embodiment of thepresent invention, addition and removal may be accomplished fromopposite sides of the package 10 c using the transparent covers 12 and30. For example, material may be added, when a frequency reduction isneeded, from the top side using the laser 1 to cause material to beadded to electrode 22. Material may be removed from the electrode 24,when a frequency increase is needed, from the bottom side using laser 2.In this way, gravity may be used to assist the tuning process.

[0020] While an embodiment is illustrated using laser irradiation, otherforms of exposure may be used including energy exposure such as heatexposure and other types of irradiation, such as infrared irradiation,may be applied through the package to tune a resonator.

[0021] While the present invention has been described with respect to alimited number of embodiments, those skilled in the art will appreciatenumerous modifications and variations therefrom. It is intended that theappended claims cover all such modifications and variations as fallwithin the true spirit and scope of this present invention.

What is claimed is:
 1. A method comprising: exposing a film bulkacoustic resonator through a cover; and changing the frequency of saidresonator through said exposure.
 2. The method of claim 1 whereinexposing a film bulk acoustic resonator includes exposing the resonatorto irradiation.
 3. The method of claim 2 wherein exposing to irradiationincludes exposing to the resonator laser irradiation.
 4. The method ofclaim 1 including exposing the resonator through a transmissive cover.5. The method of claim 4 including exposing the resonator through atransparent cover.
 6. The method of claim 4 including exposing theresonator through a pair of opposed covers.
 7. The method of claim 1including increasing the frequency of the resonator.
 8. The method ofclaim 1 including decreasing the frequency of the resonator.
 9. Themethod of claim 1 including heating a material on said cover and causingsaid material to be deposited on said resonator.
 10. The method of claim1 including enabling the addition or subtraction of material from saidresonator to increase or decrease the frequency of said resonator.
 11. Afilm bulk acoustic resonator comprising: a first electrode; apiezoelectric film; a second electrode, said piezoelectric film beingpositioned between said first and second electrodes; and a packagesurrounding said first and second electrodes and said piezoelectricfilm, said package to enable the frequency of said resonator to bealtered from outside said package.
 12. The resonator of claim 11 whereinsaid package includes a cover to enable exposure of the interior of saidpackage from outside said package.
 13. The resonator of claim 11 whereinsaid package enables irradiation of the interior of said package. 14.The resonator of claim 11 including a cover to enable light to passthrough said cover.
 15. The resonator of claim 11 including a cover thatis substantially transparent.
 16. The resonator of claim 11 including apair of covers on opposite sides of said piezoelectric film, said coversto enable exposure of the interior of said package.
 17. The resonator ofclaim 16 wherein both of said covers are substantially transparent. 18.The resonator of claim 12 including a layer of material over one of saidelectrodes, said layer of material being removable by exposure toirradiation through said cover from outside said package.
 19. Theresonator of claim 12 including material formed on said cover over oneof said electrodes, said material being heatable to cause said materialto be deposited on one of said electrodes.
 20. The resonator of claim 12wherein said cover is arranged to enable irradiation of one of saidelectrodes to either add or remove material from said electrode.
 21. Afilm bulk acoustic resonator comprising: a first electrode; apiezoelectric film; a second electrode, said piezoelectric film beinglocated between said first and second electrodes; and a packagesurrounding said first and second electrodes and said piezoelectricfilm, said package including a light transmissive cover to enableexposure of the interior of said package to change the frequency of saidresonator from outside said package.
 22. The resonator of claim 21wherein said cover is substantially transparent.
 23. The resonator ofclaim 21 including a pair of covers on opposite sides of saidpiezoelectric film, said covers to enable exposure of the interior ofsaid package.
 24. The resonator of claim 23 wherein both of said coversare substantially transparent.
 25. The resonator of claim 21 including alayer of material over one of said electrodes, said layer of materialbeing removable by exposure to irradiation through said cover fromoutside said package.
 26. The resonator of claim 21 including materialformed on said cover over one of said electrodes, said material beingheatable from outside said package to cause said material to bedeposited on one of said electrodes.
 27. The resonator of claim 26wherein said material is thermally decomposable.
 28. The resonator ofclaim 21 wherein said cover is arranged to enable irradiation of one ofsaid electrodes to remove material from said electrode.
 29. Theresonator of claim 21 wherein said cover is arranged to enableirradiation of one of said electrodes to add material to said electrode.